The present invention relates generally to a base station which may be moved from one location to another to meet changing geographic demands for trunked radio communication service.
Users in a trunked radio communication system typically share a prescribed number of communication channels. Access to the channels is coordinated by a central trunking processor, usually located within a fixed base station.
An exemplary fixed base station 98 is illustrated in FIG. 1. The base station 98 comprises a transmitting antenna 100 connected to a transmitting combiner 102 and a receiving antenna 106 connected to a receiving multicoupler 104. The transmitting combiner 102 and receiving multicoupler 104 service a plurality of RF duplex repeater modules, one of which is denoted by 108. Each repeater module is serviced by a microprocessor based control circuit 110 (referred to as a "control shelf" or CS), in association with another microprocessor based control circuit 112 (referred to as a "trunking card" or TC). The plurality of trunking cards are connected through bidirectional link to a primary site controller 114. The primary site controller 114, in turn, provides connection to a public service switched telephone network 120, a dispatch console 118, and/or a multisite controller 116. The multisite controller 116 provides access to other fixed base stations 98.
In operation, a channel assignment is initially requested by a calling radio unit which transmits a digital request signal over a digital control channel to the base station 98. The base station 98 ascertains whether a channel is available, and if so, grants the requesting mobile permission to use a working channel by passing channel assignment messages over the digital control channel to the requesting mobile. Upon receiving the assignment messages, the requesting radio tunes its receiver to the proper working channel. Any radio specified in the requesting radio's request message (the "called" radios) will also tune to the proper working channel and unmute their radios. Communication then proceeds between the requesting radio and any other "called" radio on the assigned working channel. During the call, a corresponding repeater module 108 within the base station 98 is allocated to the call.
The use of a digital control channel and associated digital data protocol provides a number of benefits. For instance, the use of a digital data protocol enables a central dispatcher to selectively transmit a message to an individual's mobile radio. The dispatcher may also transmit a message to a predefined group (or fleet) of mobile radios, such that each member within the group simultaneously receives the message. Moreover, in a digital system of the type described, communication security may be provided through the encryption of the digital data messages. A discussion of various special features typically provided in trunked radio communication systems may be found in commonly assigned U.S. Pat. No. 4,905,302 to Childress et al., which is incorporated herein in its entirety.
As one can readily appreciate, a trunked radio communication system of the type described above typically employs an extensive technical and administrative infrastructure. This has limited the application of this technology to certain users which have a projected long-term need for this service and the substantial financial resources to support the service. Governmental agencies are entities which meet this description. Commonly, for instance, a public service trunked (PST) radio service provides service to various employees of a city's fire and police departments. Private entrepreneurs may also provide trunked radio communication at one or more sites within a given geographic area for use by one or more independent business entities using special mobile radios (SMR). But even these applications usually entail a contractual commitment by the private entrepreneur to provide trunked service on a relatively long-term basis.
In contrast, short-term operations (such as disaster relief in remote areas) have not employed traditional trunked radio communication systems because there usually is not sufficient time to install and properly test the system. Other short term events (such as concerts, conventions, sporting events, and festivals) may provide ample time to install the system, but it is rarely economically feasible to do so for such a limited use. Moreover, there may be various administrative hurdles obstructing quick installation of a traditional trunked radio communication system, such as the need to ensure that the fixed range of duplex channels used by the fixed base station do not interfere with any preexisting radio communication service in the area. As a result, system providers have relied on "conventional" (non-trunked) repeaters to provide wireless communication service in these short-term applications, or dedicated communication lines.
However, the use of conventional repeater stations has notable drawbacks. The conventional repeaters typically operate in simple analog mode on a single transmit frequency and a single receive frequency, and could not provide message privacy through data encryption. This is especially a problem when police operations beyond the range of a fixed base station demand security against eavesdroppers. Furthermore, in conventional repeater stations, users can not selectively communicate messages to and from individuals or groups of individuals. It was therefore difficult to exclude extraneous messages, causing confusion among the users--often in crisis situations.
It is therefore an objective of this invention to provide trunked radio communication services for applications which have been traditionally excluded from this service. It is a more specific objective of this invention to provide a base station which may be quickly and economically deployed at a scene to provide trunked radio communication.